These teachings relate generally to wireless communications systems and methods and, more specifically, relate to cellular wireless communications systems and to techniques for a mobile station (MS), also referred to herein as user equipment (UE), to transition from one cell to another.
The following abbreviations are herewith defined, at least some of which are referred to in the ensuing description of the prior art and the preferred embodiments.
Reference can also be made to 3GPP TR 21.905, V4.4.0 (2001-10), Third Generation Partnership Project; Technical Specification Group Services and System Aspects; Vocabulary for 3GPP Specifications (Release 4), as well as to ETSI TR 101 748, V8.0.0 (2000-05), Digital cellular telecommunications system (Phase 2+); Abbreviations and acronyms (GSM 01.04 version 8.0.0 release 1999).
By way of introduction, reference is made to FIG. 1 for the ensuing description of a conventional inter-system handover from UTRAN to GSM. The UE 1 receives the GSM neighbor cell parameters from the RNC of the UTRAN 3 either in a SYSTEM INFORMATION BLOCK or in a MEASUREMENT CONTROL message. These parameters are required in order to be able to measure candidate GSM cells. Based on the measurement report from the UE 1, including GSM measurements, the RNC of the UTRAN 3 makes a handover decision. After resources have been reserved from the GSM BSS 2, via MSC 4, the RNC 3 sends an Inter-System Handover Command message (now also referred to as a HANDOVER FROM UTRAN COMMAND, as shown in FIG. 2), that also carries an embedded HANDOVER COMMAND of the GSM system. At this point, the GSM RR protocol in the UE 1 takes control and sends a GSM-specific HANDOVER ACCESS message to the GSM BSC. After a successful completion of the handover procedure, the GSM BSS 2 initiates resource release from the UTRAN 3. In response, the UTRAN 3 releases the radio connection and removes all context information for the UE 1.
Referring to FIG. 2, a more detailed description of the inter-system UTRAN to GSM handover is now provided, as specified in 3GPP TS 25.303, chapter 6.4.11. For CS domain services the UTRAN to GSM inter-RAT Handover procedure is based on measurement reports from the UE 1, but initiated from the UTRAN 3. A HANDOVER FROM UTRAN COMMAND is sent using acknowledged data transfer on the Dedicated Control Channel (DCCH). The UE transition from the UTRAN Connected Mode starts when the HANDOVER FROM UTRAN COMMAND is received. The transition to the GSM Dedicated mode, which may also be referred to as the GSM Connected mode, is finished when the HANDOVER COMPLETE message is sent from the UE 1.
The UTRAN 3 sends a RELOCATION REQUIRED to the CN/AS. This message contains information needed for the GSM system to be able to perform a handover (e.g. serving cell, target cell). Some parts of this information (e.g., MS classmark) are obtained at the setup of the RRC Connection and are stored in the CN.
The CN/AS sends a HANDOVER REQUEST message to BSC-RR 2 allocating the necessary resources to be able to receive the GSM UE 1, and acknowledges this by sending HANDOVER REQUEST ACKNOWLEDGE to the CN/AS. The HANDOVER REQUEST ACKNOWLEDGE contains a GSM-RR message (HANDOVER COMMAND) with all radio-related information that the UE 1 requires for the handover.
The CN/AS then sends a RELOCATION COMMAND (type UTRAN-to-BSS HARD HANDOVER) to the UTRAN 3 to start the execution of the handover. This message contains a GSM-RR message (HANDOVER COMMAND) with all the information needed for the UE 1 to be able to switch to the GSM cell and perform a handover to GSM.
Upon receipt of the HANDOVER FROM UTRAN COMMAND message in the UE 1, the UE-RRC entity forwards the GSM-RR message (HANDOVER COMMAND) to the MS-RR entity. To release the UTRAN resources the MS-RR entity requests the UE-RRC entity to release the RRC connection locally. The UE-RRC entity then locally releases the resources on the RLC, MAC and physical layers of the UE 1.
After having switched to the assigned GSM channel received in the HANDOVER FROM UTRAN COMMAND, the GSM MS sends HANDOVER ACCESS in successive layer 1 frames, just as it typically would have done for a conventional GSM handover initiation.
When the BSC-RR 3 has received the HANDOVER ACCESS it indicates this to the CN/AS by sending a HANDOVER DETECT message. The BSC-RR sends a PHYSICAL INFORMATION message to the GSM MS in unacknowledged mode that contains various fields of physical layer-related information allowing a proper transmission by the MS.
After layer 1 and 2 connections are successfully established, the GSM MS returns the HANDOVER COMPLETE message. The CN/AS is then able to release the UTRAN resources that were used for the UE 1 in the UTRAN Connected Mode. The CN/AS send an IU RELEASE COMMAND to the UTRAN, after which UTRAN can release all network resources from the RLC, MAC and the physical layer. When the release operation is complete, a IU RELEASE COMPLETE message is sent to the CN/AS.
The 3GPP 33.102 specification introduces the UMTS security context. The UMTS security context is used in the Release 1999 (R99) and later UMTS and GSM system releases. Chapter 4.3.2.a of 3GPP 24.008 describes a technique whereby ciphering keys for both GSM and UMTS systems are calculated during authentication. Authentication in the UMTS system guarantees the presence of ciphering keys for both systems.
The inter-system handover (HO) between two cellular system is described in 3GPP 25.331, 04.18 and 05.08 specifications, and in 3GPP 25.303. chapter 6.4.11, Inter-RAT Handover: UTRAN to GSM/BSS, CS domain services. The GSM-RR message that is discussed in the specification 3GPP 04.18, chapter 9.1.15 is the HANDOVER COMMAND. This command is delivered within the HANDOVER FROM UTRAN COMMAND, as is discussed in 3GPP 25.331, chapter 10.2.15.
In the UMTS to GSM inter-system handover the radio system changes from the UMTS radio system to the GSM radio system while an already established user data connection or link, such as a voice data or a circuit-switched data connection, continues after the handover. During this type of inter-system handover it is important that data ciphering or encryption continues uninterrupted in order to guarantee the security goals of 3GPP 33.102. In order for this to occur the GSM ciphering must begin immediately with the first transmitted data frame when handing over to the GSM system. However, the inventors have realized that the specification 3GPP 04.18, as currently written, makes it impossible to immediately initiate the use of GSM ciphering during an inter-system handover to the GSM system.
More specifically, 3GPP 04.18, chapter 3.4.4.1, states in part:
Optionally a cipher mode setting. In that case, this ciphering mode has to be applied on the new channel. If no such information is present, the ciphering mode is the same as on the previous channel. In either case the ciphering key shall not be changed. The HANDOVER COMMAND message shall not contain a cipher mode setting IE that indicates xe2x80x9cstart cipheringxe2x80x9d unless a CIPHERING MODE COMMAND message has been transmitted previously in this instance of the dedicated mode: if such a HANDOVER COMMAND message is received it shall be regarded as erroneous, a HANDOVER FAILURE message with cause xe2x80x9cProtocol error unspecifiedxe2x80x9d shall be returned immediately, and no further action taken. In the case of UTRAN to GSM handover, the HANDOVER COMMAND message, which is sent transparently via RNC from BSS to the mobile station, shall always contain the cipher mode setting IE. In the case of CDMA2000 to GSM handover, the HANDOVER COMMAND message, which is sent transparently via RNC from BSS to the mobile station, shall always contain the cipher mode setting IE.
That is, the GSM system does not permit the use of ciphering without first receiving a CIPHERING MODE COMMAND. However, the CIPHERING MODE COMMAND cannot be sent before the HANDOVER COMMAND, as no GSM connection yet exists.
Stated another way, in the case where there is an inter-system handover from UTRAN to GSM, it is not possible to send any commands before the HANDOVER COMMAND, as no GSM connection exists prior to the HANDOVER COMMAND. This being the case, ciphering cannot continue in an uninterrupted fashion during the inter-system handover to GSM, and the potential thus exists for a third party to receive a voice or data transmission that is being sent unciphered.
It is noted that it is possible to send other commands within the UTRAN specific messages. At first glance it might appear that one could readily send both the CIPHERING MODE COMMAND and the HANDOVER COMMAND transparently. However, this would be a complex approach, and would also require that a significant amount of changes be made to the specification and to the network.
It should further be noted that the current specification states that in case of a UTRAN to GSM handover the HANDOVER COMMAND shall always contain the cipher mode setting IE, but at the same time it is said that IE shall not contain a xe2x80x9cstart cipheringxe2x80x9d indication.
The foregoing and other problems are overcome, and other advantages are realized, in accordance with the presently preferred embodiments of these teachings.
Disclosed is a mobile station and a mobile station executed method for handing over from a current system to a target system. The method includes triggering the handover of the mobile station from the current system to the target system by receiving a current system-specific handover command from the current system, where the target system is one that requires the mobile station to initiate the use of data ciphering oil in response to receiving a specific command from the target system to initiate the use of data ciphering. A further step starts the use of data ciphering after handing over to the target system in response to receiving a target system-specific handover command that was sent transparently to the mobile station with the current system-specific handover command, where the target system-specific handover command has an information element set for indicating that ciphering is to be started. In the preferred embodiment the current system-specific handover command is a HANDOVER FROM UTRAN COMMAND, and the target system-specific handover command is a HANDOVER COMMAND with a cipher mode settings IE that indicates xe2x80x9cstart cipheringxe2x80x9d.
Stated another way, what is disclosed is a method for performing an inter-system handover. The method triggers the handover of the mobile station from a UTRAN system to a GSM system by sending a HANDOVER FROM UTRAN COMMAND message from the UTRAN system and, for the case where a HANDOVER COMMAND is sent transparently with the HANDOVER FROM UTRAN COMMAND, where the HANDOVER COMMAND has a cipher mode settings information element set for indicating that ciphering is to be started, beginning data ciphering immediately after handing over to the GSM system such that the first data frame transmitted by the mobile station in the GSM system is a ciphered data frame. For the case where there is a pre-existing UTRAN system ciphered data connection when the HANDOVER FROM UTRAN COMMAND message is received, continuing without interruption the use of data ciphering with the data connection after handing over to the GSM system.
The use of the teachings of this invention does not require any modifications to be made to the presently specified HANDOVER COMMAND, and thus has a minimal impact on already defined messaging and signaling formats and protocols.